1. Field of the Invention
The present invention relates to an image forming apparatus that has a fixing device for fixing unfixed toner to a sheet bearing a toner image by heating and melting it while the sheet is passed through a nip between a pair of heated rollers or between a heating belt and a roller.
2. Description of the Related Art
For image forming apparatuses such as copiers and printers, attention has been, in recent years, focused on belt methods enabling a smaller heat capacity to be set due to demands of shortening a warm-up time and saving power in a fixing device for fixing a toner image to a sheet and other demands. Attention has also been, in recent years, focused on an electromagnetic induction heating method (IH) having a possibility of quick heating and highly efficient heating, and a multitude of products combining electromagnetic induction heating and belt methods have been commercialized from the viewpoint of energy saving upon fixing a color image. In the case of combining a belt method and the electromagnetic induction heating, the arrangement of an electromagnetic induction device outside a belt is frequently employed (so-called external IH) due to easy layout of a coil and cooling and, further, due to a merit of being able to directly heat the belt.
In the above electromagnetic induction heating method, various technologies have been developed to prevent an excessive temperature increase in a sheet non-passage area in consideration of a sheet width (paper width) passed through the fixing device. Particularly, the following first and second technologies are known as size switching means in the external IH.
In the first technology, a magnetic member is divided into a plurality of pieces, which are arranged in a sheet width direction, and some of the magnetic member pieces are moved toward or away from an exciting coil in accordance with the size of a sheet to be passed (paper width). In this case, heating efficiency decreases by moving the magnetic member pieces away from the exciting coil in sheet non-passage areas, and the amount of heat generated is thought to be less than in an area corresponding to a sheet with a minimum paper width.
In the second technology, other conductive members are arranged outside a minimum paper width range in a heating roller and the positions thereof are switched between those inside and outside the extent of a magnetic field. According to the second technology, the conductive members are first located outside the extent of the magnetic field to heat the heating roller by electromagnetic induction. If the temperature of the heating roller rises to the vicinity of a Curie temperature, the conductive members are moved into the extent of the magnetic field, whereby magnetic fluxes are caused to leak from the heating roller outside the minimum paper width range to prevent excessive temperature increases.
In order to further increase productivity, the size switching means of the above first and second technologies need an effect of suppressing excessive temperature increases more than at present. For example, in order to increase the excessive temperature increase suppressing effect more than at present in the second technology, it is thought to be good to increase the area of the conductive members for shielding magnetism more than at present.
However, if the area of the conductive members is excessively increased, it becomes difficult to completely retract the conductive members from the extent of the magnetic field. Even if most of the conductive members can be retracted out of the extent of the magnetic field, there is a possibility that the remaining parts adversely affect the magnetic field. Therefore, there is a limit in enlarging the area of the conductive members even in order to increase the excessive temperature increase suppressing effect.